The
Moon (Latin: Luna) is Earth's
only natural satellite, and the fifth largest moon in the Solar
System. The average centre-to-centre distance from the Earth to the
Moon is 384,403 kilometres (238,857 mi),
which is about 30 times the diameter of the Earth. The Moon has a diameter
of 3,474 kilometres (2,159 mi)- slightly more than a quarter that of the Earth. This means that
the volume of the Moon is close to 1/50th that of Earth. The
gravitational pull at its surface is about 1/6th of Earth's.
The Moon makes a complete orbit around the Earth every 27.3 days, and the
periodic variations in the geometry of the Earth–Moon–Sun
system are responsible for the lunar phases that repeat every 29.5 days.

Man
on the Moon - Buzz Aldrin, Apollo landing

The
Moon is the only celestial body to which humans have travelled and upon
which humans have landed. The first artificial object to escape Earth's
gravity and pass near the Moon was the Soviet
Union's Luna 1, the first artificial object to impact the lunar
surface was Luna 2, and the first photographs of the normally occluded far
side of the Moon were made by Luna 3, all in 1959. The first spacecraft to
perform a successful lunar soft landing was Luna 9, and the first unmanned
vehicle to orbit the Moon was Luna 10, both in 1966. The United
States (U.S.) Apollo program achieved the only manned missions to
date, resulting in six landings between 1969 and 1972. Human exploration
of the Moon ceased with the conclusion of the Apollo program, although, as
of 2007, several countries have announced plans to send either people or
robotic spacecraft to the Moon.

Name
and etymology

Unlike
the moons of other planets, the moon of the Earth has no proper English
name other than "the Moon" (capitalized).

The
word moon is a Germanic word, related to Latin mensis;
it is ultimately a derivative of the Proto-Indo-European root me-,
also represented in measure (time), with reminders of its
importance in measuring time in words derived from it like Monday, month
and menstrual. In English, the word moon exclusively meant
"the Moon" until 1665, when it was extended to refer to the
recently-discovered natural satellites
of other planets. The Moon is occasionally referred to by its Latin name, Luna,
in order to distinguish it from other natural satellites, with a related
adjective lunar, and an adjectival prefix seleno- or suffix -selene
(from the Greek deity Selene).

Ocean
tides

Earth’socean
tides are initiated by the tidal force of Moon’s gravity and are
magnified by a host of effects in Earth’s oceans. The gravitational
tidal force arises because the side of Earth facing the Moon (nearest it)
is attracted more strongly by the Moon’s gravity than is the center of
the Earth and—even less so—the Earth’s far side. The gravitational
tide stretches the Earth’s oceans into an ellipse—with the Earth in
the center. The effect takes the form of two bulges—elevated sea
level—relative to the Earth: one nearest the Moon and one farthest from
it. Since these two bulges rotate around the Earth once a day as it spins
on its axis, ocean water is continuously rushing towards the ever-moving
bulges. The effects of the two bulges and the massive ocean currents
chasing them are magnified by an interplay of other effects; namely
frictional coupling of water to Earth’s rotation through the ocean
floors, inertia of water’s movement, ocean basins that get shallower
near land, and oscillations between different ocean basins. The magnifying
effect is a bit like water sloshing high up the sloped end of a bathtub
after a relatively small disturbance of one’s body in the deep part of
the tub.

Gravitationalcoupling
of the Moon to the tidal bulges of Earth’s oceans, which slightly lag in
phase and don’t point precisely towards the Moon, drains kinetic energy
from the Moon’s orbital motion around the Earth. This results in a 3.8
cm yearly increase in the distance between the two bodies. The Moon will
continue to move slowly away from the Earth until the tidal effects
between the two are no longer of significance, whereupon the Moon's orbit
will stabilise.

The
Moon landing

Lunar
surface

Two
sides of the Moon

The
Moon is in synchronous rotation, meaning that it keeps nearly the same
face turned towards the Earth at all times. Early in the Moon's history,
its rotation slowed and became locked in this configuration as a result of
frictional effects associated with tidal deformations caused by the Earth.

Long
ago when the Moon spun much faster, the Moon's tidal bulge preceded the
Earth-Moon line because the Moon couldn't "snap back" its bulges
quickly enough to keep its bulges in line with Earth. The rotation swept
the bulge beyond the Earth-Moon line. This out-of-line bulge caused a
torque, slowing the Moon spin, like a wrench tightening a nut. When the
Moon's spin slowed enough to match its orbital rate, then the bulge always
faced Earth, the bulge was in line with Earth, and the torque disappeared.
That's why the Moon rotates at the same rate as it orbits and we always
see the same side of the Moon.

Actually
we see a little more than half a moon. Small variations resulting from the
eccentricity of the lunar orbit, termed librations, allow about 59% of the
lunar surface to be viewed from Earth.

The
side of the Moon that faces Earth is called the near side, and the
opposite side the far side. The far side should not be confused with the
dark side, which is the hemisphere that is not being illuminated by the
Sun at a given moment (this may be the side facing the Earth, as it is
once a month during the New Moon phase). The far side of the Moon was
first photographed by the Soviet probe Luna 3 in 1959. One distinguishing
feature of the far side is its almost complete lack of maria.

Earth's Satellite

Earth has only one natural satellite, the Moon.
But

thousands of small artificial satellites have also
been placed in orbit around the Earth.

Asteroids
3753
Cruithne and 2002
AA29 have complicated orbital relationships with
the Earth; they're not really moons, the term
"companion" is being used. It is somewhat
similar to the situation with Saturn's
moons Janus
and Epimetheus.